Field of the Invention and Related Art Statement
The present invention relates to a piezoelectric vibrator comprising a piezoelectric vibrating plate and a pair of electrodes applied on opposite surfaces of the piezoelectric vibrating plate, and more particularly to a piezoelectric vibrator for generating dominantly overtone vibrations in a thickness-slide mode.
In piezoelectric vibrators, there are various modes such as flexure vibration, thickness-extensional vibration and thickness-shear vibration. In order to generate a vibration frequency higher than megahertz order, it is generally used the thickness-shear vibration mode. A fundamental frequency of the thickness-shear vibration is inversely proportional to a thickness of a piezoelectric vibrating plate made of piezoelectric material. For instance, in case of using a quartz vibrating plate of AT-cut, a fundamental frequency of 10 MHz is obtained by using a vibrating plate having a thickness of 0.167 mm and a fundamental frequency of 30 MHz is generated by using a vibrating plate having a thickness of 0.056 mm. Therefore, when it is required to provide a piezoelectric vibrator for generating a higher frequency, it is necessary to use the vibrating plate having a smaller thickness. However, it is practically difficult to manufacture the vibrating plate having a thickness smaller than 0.1 mm with high precision and high yield. Therefore, it has been general to utilize overtone, for example, third overtone in order to obtain a frequency higher than 20 MHz.
In general, the piezoelectric vibrator may oscillate at either one of the fundamental frequency and the third overtone frequency. Therefore, in order to vibrate the piezoelectric vibrator at a desired overtone frequency, it is usual to adjust variable coil or capacitor provided in an oscillator circuit having a frequency selecting property. This will be explained more in detail with reference to FIG. 1.
FIG. 1 is an equivalent circuit of a piezoelectric vibrator 1 and an oscillator circuit 2 connected to the piezoelectric vibrator. Across the piezoelectric vibrator 1 there are connected serially an effective load resistance 3 and an effective load capacitance 4 of the oscillator circuit 2. In order to vibrate the piezoelectric vibrator 1 positively, it is required to make the load resistance 3 sufficiently large as compared with a series resonance resistance of a series circuit composed of an impedance of the piezoelectric vibrator 1 and the load capacitance 4, the serial resonance resistance being termed as quivalent series resistance. Therefore, in order to vibrate the piezoelectric vibrator 1 of the thickness-shear mode at ab overtone frequency in a stable manner, an impedance of the piezoelectric vibrator 1 at the overtone frequency should be sufficiently smaller than that at the fundamental frequency.
However, in the known piezoelectric vibrators, a difference between the impedance at the overtone frequency and the impedance at the fundamental frequency is small. For instance, impedance of piezoelectric vibrators having square vibrating plates of 8 mm.times.8 mm and thickness of 0.21 mm at the fundamental frequency of 8 MHz and the third overtone frequency of 24 MHz are shown in the following Table 1.
TABLE 1 ______________________________________ Impedance at Impedance at fundamental third overtone No. frequency (.OMEGA.) frequency (.OMEGA.) ______________________________________ 1 26.1 15.6 2 26.9 15.8 3 26.8 25.4 4 21.5 19.2 5 27.2 17.3 6 20.6 17.4 7 24.7 22.1 8 26.4 16.4 9 29.2 30.1 10 20.0 17.7 ______________________________________
As can be seen from the above Table 1, the impedance of the piezoelectric vibrator at the fundamental frequency and the impedance at the third overtone frequency differ from each other only slightly. Therefore, in the oscillator circuit including the known piezoelectric vibrator, the third overtone becomes unstable to stop the overtone vibration and begins to oscillate at the undesired fundamental frequency. Particularly, when the overtone is utilized in conjunction with C-MOS inverter and TTL inverter which have been developed recently, the piezoelectric vibrator could no more vibrate at a desired overtone frequency if characteristics of the vibrator and oscillation circuit change slightly.